1. Field of the Invention
The present invention relates to a bridgeless power factor correction circuit, and more particularly, to a bridgeless power factor correction circuit that corrects a power factor by complementarily switching two switches on and off according to phases of input power without using a rectifier bridge diode.
2. Description of the Related Art
In recent years, as harmonics regulations have been tightened worldwide, the use of power factor correction circuits in electronic products has become common and required.
In general, the power factor correction circuit is divided into a passive power factor correction circuit and an active power factor circuit. The passive power factor correction circuit is configured by appropriately designing a passive line filter composed of an inductor and a capacitor at a power input side so as to attenuate harmonic components of power current. The passive power factor correction circuit has a simple configuration and is manufactured at low cost. However, it is difficult to stabilize a voltage since the size of an output voltage varies according to the size of an input power voltage. Further, since the passive power factor correction circuit needs to be designed according to commercial power frequency, it increases in size and volume significantly.
The active power factor correction circuit is configured by applying a generally known boost converter. The active power factor correction circuit has a power factor almost close to 1 and can output stable direct current power regardless of a variation in input voltage. However, since the active power factor correction circuit uses a switching method, a configuration circuit becomes complex to increase the unit cost, and it becomes difficult to control the circuit.
Therefore, in order to satisfy the harmonic regulations that have been tightened worldwide, most of the high power products are using active power factor correction circuits.
FIG. 1 is a circuit diagram illustrating a general active power factor correction circuit.
As shown in FIG. 1, an active power factor correction circuit using a general boost converter uses bridge diodes BD at an input terminal. The bridge diodes BD cause high conduction loss of approximately 2 to 3% of all power capacity. Therefore, studies on various types of bridgeless power factor correction circuits have been conducted these days.
FIG. 2A is a circuit diagram illustrating a general active power factor correction circuit. FIG. 2B is a waveform diagram illustrating a main part of the general active power factor correction circuit.
Referring to FIG. 2A, as described above, a general bridgeless power factor correction circuit, which is an improvement on the general active power factor correction circuit, is shown.
The above-described general bridgeless power factor correction circuit detects a current flowing through boost inductors L1 and L2 through a detection resistor Rsense. That is, the detection resistor Rsense is connected between diodes D1 and D3 and a ground terminal of the circuit to detect the current flowing through the inductors.
Then, a sine wave having the same phase as the input voltage is obtained from the detected current flowing through the inductors by using a correction circuit. At the same time, an output voltage is detected by using resistors R1 and R2, and the sine wave obtained by detecting the current flowing through the inductors is multiplied by appropriate gain according to a value of the detected output voltage.
The signal, formed as described above, is compared with a triangle wave to generate a gate signal for driving a switch. That is, the gate signal of the switch is generated by detecting the current flowing through the inductors and the output voltage. At this time, the gate signal of the above-described switch is used to turn on or off the first and second switches M1 and M2 at the same time.
FIG. 2B is a waveform diagram illustrating a main part of the general bridgeless power factor correction circuit.
Referring to FIGS. 2A and 2B, the difference between forward voltage drop across the diodes D1 and D2 and voltage drop the sensing resistor Rsense and on-resistance of the switches M1 and M2 and backward voltage drop across the diodes D3 and D4 is applied to the first and second inductors L1 and L2.
That is, when the switches M1 and M2 are turned on at the same time, one of the first and second inductors L1 and L2 operates as a boost inductor according to phases of the input voltage. When the switches M1 and M2 are turned on, voltage drop across the switches M1 and M2 becomes smaller than voltage drop across the diodes D1 to D4, so that the drop voltage across the diodes D1 to D4 is applied to the other inductor. Even though a very small voltage obtained by subtracting the voltage drop across the switches M1 and M2 from the voltage drop across the diodes D1 to D4 is applied to the other inductor, since the voltage is applied for a long period of time, that is, a half cycle of the input voltage of approximately 8.3 msec, a considerable amount of current continues to flow through the inductors, which is supposed to operate at very fast switching frequency, during the half cycle of the input voltage.
FIG. 3 is a current waveform diagram illustrating a general bridgeless power factor circuit.
Referring to FIG. 3, it can be seen that the shape of the input current is distorted due to a reverse current of the inductor. This causes a reduction in power factor and generation of high harmonic components.